1. Field of the Invention
The present invention relates to a lens system for use in an optical information device in which a laser beam is employed to record and reproduce information.
2. Description of the Relevant Art
Various optical information devices such as optical disc systems, magnetooptic systems, optical card systems, and the like employ an optical lens system comprising a number of lenses for focusing a divergent light beam emitted from a light source such as a semiconductor laser onto the recording surface of a recording medium.
Such lens systems for use in optical information devices are required to have various aberrations such as spherical aberration, coma, etc. to be properly corrected and also to be small in size and light in weight so that a lens actuator for driving the lens system has a higher level of performance for focusing and tracking In addition, it is important that lens systems be less costly to manufacture in order to make optical information devices incorporating the lens systems available relatively inexpensively in the market. Known optical lens systems include a collimator lens for rendering diverging light rays parallel and several homogeneous glass lenses for focusing the light beam onto a recording surface. For example, an optical lens system including a single aspherical lens or a single lens with a radial refractive index distribution is known as disclosed in U.S. Pat. Nos. 4,613,212, 4,457,590, and 4,701,032.
Generally, optical information processing systems require that information recording tracks or bits be closely spaced in order to record and reproduce as large an amount of information as possible. Therefore, a lens in such systems, particularly an objective lens, needs to have a diffraction limit performance. The diffraction limit performance of a lens is generally determined by the wavelength of light passing therethrough and a numerical aperture (NA) of the lens. The greater the NA of a lens, the smaller the spot of a light beam focused by the lens. It is therefore important for lenses for use in optical information recording and reproducing systems to have a large NA. If conventional optical lens systems have an NA or 0.5 or more, then the number of lenses required is increased, aberrations are difficult to correct, and strict tolerances are required.